The can making industry is characterized by a line of products which are produced in very high volume, with high quality control standards, but where the profit contribution of each unit is small. As a consequence there is a need for quality control methods which are rapid, effective and which require little judgement on the part of the operator, who may be unskilled. One of the factors which is routinely evaluated is the continuity of the enamel lining of the container. Since the enamel lining serves to protect the product from possible contamination by interaction with the metal and at the same time protects the can from deterioration, the enamel must be free from voids or defects. It is well known that discontinuities in a coating can be detected by impressing a low voltage across the coating in the presence of an electrolyte and observing whether a flow of current occurs. The electrolyte fills the voids or coating discontinuities and establishes a conductive path between the grounded metallic substrate upon which the coating is deposited and the electrode in contact with the electrolyte. Kronstein et al describe such a method in Industrial and Engineering Chemistry Volume 42, Pages 1568-72 (1950). Kronstein used paper saturated with dilute potassium nitrate as the electrolyte. The substrate was made the anode in the circuit and metallic ions were captured on the paper and subsequently precipitated to form a colored salt, thereby indicating current flow and the presence of a void.
Pipe and tank linings have been evaluated for continuity using a sponge saturated with a dilute salt solution, which is mounted on a wand. The wand is connected to one terminal of a DC power supply and the tank or pipe is connected to the other terminal. Discontinuities in the coating are located by a bell in the circuit which rings when current flow occurs.
More recently, the technique has been used in the container industry. An instrument known as the WACO Enamel Rater manufactured by the Wilkens-Anderson Company rates the lined container by measuring the current flow in milliamperes. The test requires that the container be filled to within 1/8" of the top of the can, that the electrode be lowered into the can and the test be allowed to proceed for the stipulated period, after which a measurement is made. In most instruments, the meter is of the analog type wherein the deflection of a needle is observed and a reading is estimated from the scale. In some cases a digital meter is available, but even this is subject to misinterpretation since a dead meter or a broken circuit gives a reading which is equivalent to that of a good continuously lined container. Since a properly filled container is apt to result in spillage due to the proximity of the fluid level to the top of the container, there is a definite tendency on the part of an inspector to under fill the cans. This is particularly true in a production line environment where there is pressure to keep up with the production. Where the can is under filled by even a fraction of an inch, the continuity test is unsatisfactory since the area of the container adjacent to the rim is critical, and will not be included in an under filled can. Currently available instruments fail to distinguish between a properly and improperly filled container.
Accordingly, it is an object of this invention to provide an improved continuity tester for container linings which ensures that the entire interior surface of the lined container is evaluated for continuity during a test.
It is also an object of this invention to provide an improved continuity tester which is easy to read, does not require interpolation or estimation of the output and where an open circuit, cannot be misinterpreted for a satisfactory response.
Finally, it is an object of this invention to provide an instrument which is compatible with an industrial environment, which is rugged, reliable and may be handled by a relatively unskilled operator.
The inventor is not aware of any patents which are material to the examination of the application.